In order to implement a DWDM-PON (dense wavelength division multiplexing passive optical network) optical transport network which is capable of providing a large quantity of communication service, development of a tunable light source which is tunable and modulated at a high speed becomes important.
A representative fast modulation tunable laser among such tunable light sources is a tunable laser that uses a sampled grating distributed Bragg reflector (SG-DBR). Such a SG-DBR tunable laser uses a Vernier effect to extend a tunable range. The SG-DBR tunable laser requires a complex control circuit such as a Vernier control circuit, a wavelength stabilizing optical element and a wavelength stabilizing control circuit, and a phase adjustment control circuit. If the SG-DBR tunable laser uses two ring resonate phase adjusting unit having different free spectral ranges (FSR) instead of using the Bragg grating reflector, the manufacturing process is simple. However, the difference of FSRs between two ring resonate phase adjusting units is small so that complex control may be required for a stable output wavelength.
There is an external cavity tunable laser that uses an external tunable optical element in order to tune an output wavelength of the laser, which is different from the SG-DBR tunable laser and the tunable laser using two ring resonate phase adjusting units which are monolithic integrated tunable lasers. The external cavity tunable laser is manufactured such that a polymer based planar lightwave circuit (PLC) and an optical gain chip are separately manufactured and then the two optical elements are combined to manufacture the tunable laser. The external cavity tunable laser is simply controlled in order to tune the wavelength and has an excellent productivity. The polymer based tunable external cavity laser uses a super luminescent diode (SLD) as an optical gain chip and directly modulates the optical gain chip to have a modulation speed of 2.5 Gbps.
For high speed data transmission at 10 Gbps or higher and long distance data transmission, a tunable laser which is capable of performing high speed modulation is required. An external cavity tunable laser in which a comb reflecting unit is integrated in a light source has a structure in which an optical modulating unit and an optical amplifier are integrated at the output of the laser. Therefore, such an external cavity tunable laser is suitable for the high speed modulation and long distance transmission. Since the external cavity tunable laser uses an external tunable optical element, the external cavity tunable laser may have an excellent productivity and easily control tuning of a wavelength.
The external cavity tunable laser includes a highly integrated light source and a tunable reflecting unit. However, in the external cavity tunable laser, the reflection at both edges of the highly integrated light source may deteriorate the output characteristic of the external cavity tunable laser. Therefore, in order to reduce the reflectance at both edges of the highly integrated light source, the reflectance may be reduced to be approximately 10E-4. For this reason, the external cavity tunable laser generally uses two methods including a method using an inclined waveguide and a non-reflective coating method. The external cavity tunable laser uses the non-reflective coating method to reduce the reflectance. However, it is difficult to obtain sufficiently low reflectance when using only the non-reflective coating method. Therefore, the external cavity tunable laser reduces the reflectance by inclining the output waveguide to form an angle.
However, if the external cavity tunable laser uses the inclined output waveguide in order to reduce the reflectance, an alignment axis between the tunable reflecting unit and the highly integrated light source is different from an alignment axis between an output optical fiber and the highly integrated light source. Accordingly, a width and volume of an external cavity tunable laser apparatus that uses the comb reflecting unit and the ring resonant phase adjusting unit to use a long light source may be increased due to the usage of the inclined waveguide. There is a difficulty in reducing the size of the tunable laser. If the high speed optical signal which is modulated by the optical modulating unit is directly input to the optical amplifier, the optical signal is distorted due to the saturated power of the optical amplifier, which causes problems in the high speed modulation and the long distance transmission. If an optical gain unit and the optical amplifier have the same active layer, a large optical gain and a small optical saturation input power are required. However, it is very hard to manufacture such a highly integrated light source.